The invention relates to a method for operating an asynchronous machine comprising a rotatably mounted rotor and a stator which is fixed to the housing, wherein a torque of the asynchronous machine is adjusted by specifying a desired magnetic flux of a surrounding magnetic field of the stator and specifying a desired slip between a rotational speed of the rotor and the rotational speed of the surrounding magnetic field.
The invention furthermore relates to a corresponding device for operating an asynchronous machine, which comprises a rotatably mounted rotor and a stator that is fixed to the housing, said device comprising equipment, in particular power electronics, for adjusting a torque of the asynchronous machine, wherein the equipment for adjusting the torque specifies a desired magnetic flux of a surrounding magnetic field of the stator and a desired slip between a rotational speed of the rotor and the rotational speed of the surrounding magnetic field.
The invention furthermore relates to a corresponding asynchronous machine.
Methods, devices and asynchronous machines of the type mentioned above are known from the prior art. The torque of asynchronous machines is particularly dependent on two variables, on the magnetic flux of the magnetic field surrounding the stator and on the slip between the rotational speed of the rotor and the “rotational speed” of the surrounding magnetic field. When driving the asynchronous machine by means of power electronics, the two variables are accordingly manipulated variables to be specified for achieving the desired torque. Particularly during generator operation of the asynchronous machine, a desired slip is adjusted such that a rotary frequency of the surrounding magnetic field is smaller in amount than the rotary frequency of the rotor. In so doing, it is possible using a conventional control algorithm that the desired value of the rotary frequency of the stator field is calculated to be 0 Hz while a significant current is flowing through the stator.
A limitation for the operation of power electronics or, respectively, the equipment actuating the asynchronous machine is the thermal load capacity of the switching elements or, respectively, the power switches. When operating the electrical machine by means of power electronics, a critical operating point can then exist if, in a load condition, a large portion of the current flows for an extended period of time across a single power switch such as, for example, in the case of a B6 bridge. This can, for example, occur in an electric vehicle such that the electrical machine is permanently excited when initiating a driving operation on an uphill, i.e. if a torque is required and the rotational speed of said electrical machine is, for example, still equal to zero.